The present invention relates to the fabrication of semiconductor devices and, more particularly, to a method for removing residue molding material from metal parts of plastic packages for semiconductor devices.
As is known, many semiconductor power devices (e.g., discrete transistors, integrated circuits) are contained in packages including a plastic body with terminal conductors protruding therefrom, and a metal plate. The function of the metal plate is to dissipate heat produced by the device during its operation, or to transfer this heat external the device. The metal plate is usually mounted in contact with a flat surface of an external sink of large dimensions.
For the fabrication of a device of this type, a chip of semiconductor material is fixed to the metal plate which acts as a heat-sink. A metal frame is then mounted on the plate electrically insulated therefrom. The frame, obtained by stamping from sheet metal, includes metal strips which will become the terminals of the device and interconnection bars between the strips. Thin wires are soldered to appropriate areas of the chip and to the ends of the metal strips. The resulting structure is mounted, together with other identical structures, in a special mold with one face of the heat-sink in contact with a flat surface of the mold.
Next, a plastic, such as a thermosetting epoxy resin, is introduced in a liquid state into the mold. After polymerization of the resin, a body of solid plastic is extracted from the mold. This body encloses the whole structure with the exception of a surface of the heat-sink that has remained in contact with the flat surface of the mold. Also not enclosed are portions of the metal strips and of the interconnection bars between them. These latter items are cut off in a subsequent manufacturing phase.
In many cases during the molding operation, the liquid-state resin penetrates between the heat-sink and mold due to small irregularities in the contacting surfaces. This is due, for example, to a slight curvature of the heat-sink or to the wearing of the mold. Therefore, after extraction from the mold, the package may exhibit residues of molding material, i.e., of cured resin, on the heat-sink. Similar infiltrations may occur on the metal strips with the consequent formation of residue on these also, especially near the plastic body.
The residue of plastic on the heat-sink has to be removed since it would otherwise reduce the surface of contact with an external sink, or with a metal plate of a printed circuit onto which the heat-sink is to be soldered. The residue on the terminals also has to be eliminated since it impedes the cutting off of the interconnection bars and the subsequent operation of bending the terminals.
Various methods for removing such residue of molding material are known. For example, electrical or chemical treatments are known which can be advantageously used on the terminals without being used on the heat-sinks. Sand-blasting treatments are also known which enable even thick residue to be removed from the heat-sink and from the terminals. However, because of their very energetic mechanical action, they do not lend themselves to being used in the treatment of plastic packages of small dimensions with very thin and very closely spaced terminals.
An object of the present invention is to provide a method for removing residue of molding material from metal parts of plastic packages while not mechanically stressing the parts to be treated, and can therefore also be used in the treatment of plastic packages with thin terminals.
The present invention uses laser radiation, i.e., coherent monochromatic radiation, for the controlled application of high-intensity energy. Although the use of lasers in the treatment of materials and in industrial manufacture has been known for some time, it has not before been contemplated with respect to the removal of residue of molding material from plastic packages. Laser radiation has not been used because it was most likely determined to be unsuitable for such a purpose.